Optical Trapping Separation of Chiral Nanoparticles by Subwavelength Slot Waveguides

Enantiomer separation opens great opportunities to develop the technologies of pharmaceutics, chemicals, and biomedicine, but faces daunting challenges. Here, we discover a considerable chiral-dependent trapping force to separate nanometer-scale enantiomers in a new silicon-based waveguide platform. The electromagnetic chirality gradient of strongly confined evanescent fields can be largely enhanced by the counterpropagating slot waveguides so that the resulting chiral gradient forces can shift the trapping equilibrium positions of dielectric gradient forces. Especially, there exists a transitional width for the slot waveguides to exchange the trapping equilibrium positions between two opposite enantiomers. Our thoroughly numerical investigations demonstrate that the chiral-separable slot waveguides here can offer high efficiency and feasibility of separating chiral nanoparticles, and may pave a route toward new on-chip chiral optical tweezers or optofluidic transport systems for large-scale chiral separation.

Figure 1

Optical trapping separation in the counterpropagating slot waveguides. (a) Schematic of chiral trapping by the strongly confined standing evanescent fields in the gap of slot waveguides. (b) The electric-field intensity and (c) the imaginary part of electromagnetic product (chirality) of the PTM modes. The resulting trapping force potentials and chiral-dependent trapping shifts in the gap of slot waveguides for (d) $R$ enantiomers, (e) achiral particles, and (f) $S$ enantiomers.

Figure 2

Chiral-dependent gradient forces or trapping separation versus varied chirality parameters and sizes of nanoparticles in the slot waveguides with $h=100\mathrm{nm}$, $w=0.8\mu \mathrm{m}$, and the input power of 100 mW. (a) The three gradient force components and the resulting gradient forces under $\kappa =1$, 0.5, and 0.1, respectively. (b) The degrees of trapping separation versus $\kappa$ varied from $-1$ to 1. The resulting gradient forces $F_g$ exerted on the $R$ enantiomers ($\kappa =1$) with (c) different particle sizes and (d) different operating wavelengths.

Figure 3

Magnetic-field distributions, chiral-dependent gradient forces, or trapping separation for the $R$ enantiomers ($\kappa =1$) with the radius of 12 nm under varied parameters of the slot waveguides in the wavelength of 1550 nm. (a)–(d) The magnetic-field distributions in the slot waveguides with a slot distance $h=100\mathrm{nm}$ and different widths $w$. (e) The degrees of trapping separation versus $w$ varied from $0.4\mu \mathrm{m}$ to $\text{1.2\hspace{0.17em}\hspace{0.17em}}\mu \mathrm{m}$. Note that the stars denote the discrete simulation data, and the curves indicate fitted values. (f) The chiral gradient forces $F_{g3}$, and (g) the resulting trapping forces $F_g$ under different widths $w$, when $h=100\mathrm{nm}$. (h) The resulting forces $F_g$ under different slot distances $h$, when $w=0.9\mu \mathrm{m}$.

Figure 4

2D stochastic Brownian motion simulation of chiral-selective driving and trapping in the slot waveguides for (a) $R$ enantiomers, (b) achiral particles, and (c) $S$ enantiomers. Note that the lines marked with different colors represent the trajectories of moving particles from different initial positions.